Muon Identification in DELPHI

MUON IDENTIFICATION IN DELPHI

Warning: the information on this page has not been updated since August 95! Its content is probably totally obsolete and you should ask the muon id. convenors for fresh information. Good luck!

Introduction

The muon identification with the DELPHI muon chambers (MUB in the Barrel and MUF in the End Caps) is performed with the MUFLAG package. It allows the use of various types of tags, named Very Loose, Loose, Standard and Tight. These 4 tags provide to the user different efficiency and purity levels.
This page contains only a short summary on how to use this package and what are the main results. A more detailed description of the muon tagging performance cand be found in the DELPHI note DELPHI 95-140 PHYS 565.

HOW TO USE MUFLAG

Three routines can be used to tag muons:
-1) The routine MUFLAG gives the list of all muons found in one event for a given tag :

        CALL MUFLAG(ITYPE,NMU,LMU)

    where: - ITYPE is the tag type  = 1 for the Standard tag
                                    = 2 for the Tight tag
                                    = 3 for the Loose tag
                                    = 4 for the Very Loose tag
           - NMU is the number of identified muons (up to 20)
           - LMU is a 20 element integer array, holding the PA
             identifier of all the tagged muons.

-2) The routine MUCALL gives the list of Standard, Tight and Loose muons :

        CALL MUCALL(NMUS,LMUS,NMUT,LMUT,NMUL,LMUL)

-3) The routine MUCAL2 gives the list of Standard, Tight, Loose and Very Loose muons :

        CALL MUCAL2(NMUS,LMUS,NMUT,LMUT,NMUL,LMUL,NMUVL,LMUVL)

The MUFLAG package is part of the DSTANA library.

Three external input files containing the MUB/MUF geometry and some calibration constants need to be used.
They are stored on the DELPHI_DAT area on VXCERN and SHIFT and no user assignement is needed on these platforms.

MUBGEO.DAT,
MUFGEO.DAT
MUBCAL94.DAT

MUON TAGGING CRITERIA

The muon tagging is based on the number of associated MUC layers to the muon candidate track and on the extrapolation and global chi2's of the track fit (see EMMASS note DELPHI 92-17 PHYS 157 for the exact definition of these chi2's). These chi2's are computed using specific track fit conditions for each tag.
For the Standard and Tight tags, at least one hit outside the iron must be associated to the track. For the Very Loose and Loose tags, one associated hit anywhere in the MUC is enough.
For the Loose, Standard and Tight tags, additional cuts on the global and extrapolation chi2's are applied, to gradually reduce the misidentification probability.

MUON IDENTIFICATION EFFICIENCIES

Data Samples used

To cover the full muon momentum spectrum between 3 and 45 GeV/c, various muon samples have been studied:

Muon pairs coming from Z0 decay (muon momentum close to 45 GeV/c),
Muons coming from tau lepton decay (muon momentum from 3 up to 40 GeV/c) and,
Muon pairs produced in two photons collisions (very low momentum muons produced mainly in the forward region).

Data Samples on Tapes

These samples have been selected for 92D, 93B, 93C and 94B data and MC. They are stored on IBM cassettes and you can find the list of them in the file ::

VXDEL1::VXDEL1$USER:[MUONTASK.DATLIST]DATASETS.LIST.

Muon Chambers Acceptance

Only muons produced in the MUC polar angle acceptance are studied: 0.026<|cos(theta)|<0.616 for the MUB and 0.743<|cos(theta)|<0.940 for the MUF.
They must also have a momentum greater than 3 GeV/c.

Global Results

Here are tables showing the global muon identification efficiencies obtained with the 94B2 data in MUB and MUF respectively.

   Table 1 : Muon Identification Efficiencies in MUB for 94B2 Data (in %)

          Data      |     VL    |     L     |     S     |     T     | 
    ----------------|-----------|-----------|-----------|-----------|
    Z->mu+mu-  Data | 95.7+-0.1 | 94.4+-0.1 | 84.8+-0.2 | 75.6+-0.2 |
               MC   | 96.1+-0.1 | 95.5+-0.1 | 84.6+-0.1 | 75.2+-0.2 |
    ----------------|-----------|-----------|-----------|-----------|
    tau->mu    Data | 95.6+-0.2 | 91.4+-0.3 | 79.9+-0.5 | 65.9+-0.6 |
               MC   | 96.3+-0.2 | 92.6+-0.3 | 79.7+-0.4 | 65.8+-0.6 |
    ----------------|-----------|-----------|-----------|-----------|
    gg->mu+mu- Data | 90.6+-1.0 | 85.5+-1.2 | 69.1+-1.8 | 47.1+-2.4 |
               MC   | 93.3+-0.4 | 87.2+-0.6 | 66.9+-0.9 | 47.2+-1.1 |
    -----------------------------------------------------------------

   Table 2 : Muon Identification Efficiencies in MUF for 94B2 Data (in %)

          Data      |     VL    |     L     |     S     |     T     | 
    ----------------|-----------|-----------|-----------|-----------|
    Z->mu+mu-  Data | 96.5+-0.2 | 95.9+-0.2 | 89.6+-0.3 | 77.1+-0.4 |
               MC   | 96.5+-0.1 | 95.9+-0.1 | 89.6+-0.2 | 77.9+-0.3 |
    ----------------|-----------|-----------|-----------|-----------|
    tau->mu    Data | 94.4+-0.4 | 92.4+-0.5 | 84.5+-0.7 | 67.9+-1.0 |
               MC   | 94.0+-0.4 | 92.5+-0.4 | 83.8+-0.6 | 67.7+-0.9 |
    ----------------|-----------|-----------|-----------|-----------|
    gg->mu+mu- Data | 92.2+-0.5 | 90.2+-0.6 | 77.3+-0.9 | 54.7+-1.2 |
               MC   | 92.2+-0.2 | 90.7+-0.3 | 78.1+-0.4 | 55.8+-0.6 |
    -----------------------------------------------------------------

Momentum Dependence

Figure 1 shows the momentum dependence of identification efficiency in MUB and MUF for the Standard tag (94B2 data).

Polar Angle Dependence

Figure 2 shows the polar angle dependence of identification efficiency for the Standard tag (94B2 data).

MUON MIS-IDENTIFICATION PROBABILITIES

The study of muon misidentification probability is based on tau's decaying into 3 charged pions. These events have been selected in the Barrel and Forward region with a high purity (> 95 %). The misidentification probabilities shown in the following table have been corrected for the remaining non pion background.

           Table 3 : Misidentification probability for 94B2 data (in %)

              |          Barrel         |         Forward         |
         Tag  |    Data    |     MC     |    Data    |     MC     |
        ------|------------|------------|------------|------------|
          VL  |  4.9+-0.2  |  2.9+-0.1  |  7.9+-0.6  |  6.2+-0.4  |
          L   |  1.3+-0.1  | 0.66+-0.06 |  2.3+-0.3  |  1.8+-0.2  |
          S   | 0.65+-0.07 | 0.27+-0.04 |  0.9+-0.2  |  0.7+-0.1  |
          T   | 0.42+-0.06 | 0.16+-0.03 | 0.55+-0.16 | 0.36+-0.10 |
        -----------------------------------------------------------

Last Update of this page Aug 31, 1995
F.Stichelbaut
G.Wilkinson